To handle the explosive expansion of information-communication traffic, it has been required lately and in the future to extend the capacity of the backbone optical communication network.
In optical networks, optical bandwidths are used in accordance with the dense wavelength division multiplexing (DWDM) system standardized by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). In the DWDM system, the entire available optical bandwidth is divided into narrow segments by a grid with constant width, called a wavelength grid, and optical signals in one wavelength channel are allocated within a grid spacing (ITU-T recommendation G.694.1). More specifically, the wavelength bandwidth of the optical communication (hereinafter referred to as a slot), the minimum unit of which has been 50 GHz till now, is divided into a narrow segment of 12.5 GHz. In addition, the number of wavelength slots to be allocated to each channel of the optical communication (optical path) is made variable, which makes it possible to keep the wavelength slots to be allocated to each optical path to the minimum necessary.
A new problem, however, has been caused with respect to wavelength band allocation. The problem will be described below. It is considered for example to allocate four wavelength slots to an optical path. If there are ten empty slots in the entire wavelength band of an optical fiber, and for details, two consecutive empty slots have been allocated to each of five optical bands discontinuously, the above-mentioned four wavelength slots in full width cannot be allocated to the optical path. That is to say, although there are a sufficient number of empty slots in total, consecutive empty slots cannot be ensured because each empty slot is composed of short segments. As a result, it is impossible to allocate to the optical path a wide wavelength band which allows high-capacity or long-haul communication. This is called a fragmentation of the wavelength band allocation (wavelength fragmentation), which becomes more likely to arise as the central wavelength of the optical path or the number of wavelength slots is changed repeatedly.
PTL 1 discloses an optical transmitter in which the wavelength multiplexing spacing of a transmission line (an optical fiber) is closed up by controlling the carrier frequency to solve the above-mentioned problem. The optical transmitter according to PTL 1 includes a laser light source, a signal processing circuit, a DAC (digital to analog converter), a driver, an optical modulator, and a carrier-wave frequency control circuit. Digital modulated signals output from the signal processing circuit are converted into analog modulated signals by the DAC, which are amplified by the driver. The signals are used for driving the optical modulator, which modulates the optical signal output from the laser light source. The carrier-wave frequency control circuit controls the frequency of the optical signal output from the optical modulator. This configuration enables the band utilization rate of the transmission line to improve.
PTL 2 discloses an optical network system in which segmentalized, that is, fragmented wavelength locations (wavelength slot) of the signal light are rearranged (defragmentation) so that the wavelength locations of signal light beams modulated by the same modulation scheme may lie next to each other. The optical network system according to PTL 2 includes a network controller, an optical transmitting and receiving device, an ROADM (Reconfigurable Optical Add-Drop Multiplexer) device, and an optical amplification unit. If the number of empty wavelength slots drops to below a certain value, the wavelength of the optical transmitting and receiving device is changed on the basis of the information on the empty wavelength slots in the network that is included in the network controller, and then the wavelength bandwidth of an optical amplifier of the ROADM device is expanded. Such wavelength defragmentation (rearrangement of wavelength) is repeated until signal light beams come to lie next to each other with respect to each modulation scheme. If the band used for the signal light is expanded, the signals modulated by the modulation scheme that is remarkably tolerant of the optical signal-to-noise ratio are preferentially moved to the expanded band. The configuration makes it possible to decrease the number of guard bands which prevent the interference between wavelength slots lying next to each other, and to improve the usage efficiency of the wavelength slot.
If failures occur in an optical transmission line, which is a route of optical communication in an optical network, it is necessary to restore the optical communication promptly; and therefore, an optical network system is operated that includes active optical fibers (active system optical paths) and standby optical fibers (standby optical paths) provided in the optical transmission line. The optical network system uses the active optical fibers in the normal optical communication, and the active optical fibers are promptly switched to the standby optical fiber when a communication breakdown occurs due to a failure of the active optical fiber. Some of the optical network systems are configured to use the standby optical fibers even in the normal operation. For example, PTL 3 discloses an optical transmission network in which signals for transmission having low priority are transmitted through the standby optical fibers in the normal operation. The optical transmission network according to PTL 3 includes a ring-like route in which a plurality of optical transceivers having the function for wavelength-division multiplexing are connected to each other by optical transmission devices. The optical transmission device is equipped with active optical fibers and standby optical fibers, and objects to be transmitted are prioritized. In the normal operation, main signals having high priority are transmitted through the active optical fibers, and sub-signals having low priority are transmitted through the standby optical fibers. During system failures, the sub-signal transmission is suspended, and the standby optical fiber propagates main signals. This makes it possible to transmit objects to be transmitted in the normal operation through the standby optical fibers different from the active optical fibers.